1. Field of the Invention
The present invention relates to a Mach-Zehnder modulator which modulates the intensity of laser light in response to an electric signal for of optical communications.
2. Description of the Related Art
In a Mach-Zehnder modulator of the prior art, as reported by D. M. Adams et al. (Electronics Letters 29th February 1996, vol.32, No.5 p485-486), for example, incident signal laser light is split to pass through two optical waveguides, each optical waveguide having a multiple quantum well structure (hereinafter referred to as "MQW") comprising an InGaAs well and an InGaAsP barrier is sandwiched by clad layers (having the same construction as that shown in FIGS. 1A, 1B)
When a positive modulating voltage is applied to the MQW of one branch optical waveguide and a negative modulating voltage is applied to the MQW of the other branch optical waveguide, refractive indices of the two branch optical waveguides change in the opposite senses due to the Stark effect of quantum confinement. That is, refractive index increases in one branch optical waveguide and decreases in the other branch optical waveguide.
As a result, the phase of the signal light transmitted in the optical waveguides gains in one optical waveguide and the delays in the other optical waveguide, and intensity of the output signal light can be modulated by synthesizing, i.e., combining, these signal lights.
The Mach-Zehnder modulator of the prior art, because of two branch optical waveguides having the same construction, has a problem in having different phases in the two optical waveguides requiring application of modulating voltages of opposite phases to the two optical waveguides, thus requiring a complicated drive circuit for the modulating voltage.